Carbonitriding process



Nov. 24, 1964 J. B. SAXTON 3,153,514

CARBONITRIDING PROCESS Filed April 10, 1962 60 DIRECT OUENCHED FROM I650F //v /0 4/100500:

f NACL sou/flog; m/zmq/Z/Z/m z/z/lz/jzfl/jjzz/jm/ jg;

T T WI/50F 1 \l 4 30 9: I 20 3 E Q i /0 E t U a m 3 Q 0 HOURS HELD AT /ND/CA TED TEMPERATURES AFTER 545T COOL ING FROM AUS TEN/T/Z/NG TEMPERATURE OF I650? JOHN B. SAX TON INVENTOR ATTORN EYS United States Patent 3,158,514 CARBGNII'EGDING PRSCESS John B. Sexton, Detroit, Mich, assignor to Ford Motor Company, Dearhorn, Mich, a corporation oi Delaware Filed Apr. 10, 1962, Ser. No. 186,377 1 Ciairn. (Cl. 148-165) This invention relates to a process for heat treating ferrous products and more particularly to a process for carbonitriding nodular iron and gray iron. Carbonitriding is a process employed to produce an intensely hard case upon ferrous objects and involves essentially the exposure of such a ferrous article to a temperature of at least 1000 F. in an atmosphere containing carburizing gas and a source of combined nitrogen such as ammonia. A very clear description of this process in detail is contained in the American Society for Metals Handbook, 1948 edition, at pages 696-697.

This invention is especially directed to a process for applying this carbonitriding process to articles formed of nodular iron or gray iron whereby a hard surface is produced over a core which also has the maximum possible hardness. A typical nodular iron composition suitable for the practice of this invention is tabulated below:

This invention is also applicable to any conventional gray iron composition.

To aid in understanding this invention, a single figure of drawing has been presented. This drawing is a graphical representation of hardness obtained in nodular iron specimens when quenched from an austenitizing temperature after having been held for varying lengths of time at supercritical temperatures.

The data depicted by this drawing was obtained by fully austenitizing identical samples of nodular iron and bringing these samples to a uniform temperature of 1650 F. One sample was cooled to 1550 F. and held for four hours and quenched. A second sample was cooled to 1500" F., held for four hours and quenched. A third sample was similarly treated at 1450 F. This drawing demonstrates that at quenching temperatures at least as high as 1500" F. there is obtained essentially the same hardness as that obtained from a quenching temperature of 1650 F. Contrariwise, the specimen permitted to remain at 1450 F. gave a decidedly inferior hardness.

This inferior hardness is due to the premature and unwanted decomposition of austenite into the soft constituent ferrite at 1450 F.

This work clearly indicated a course to be taken to enable a nodular iron or gray iron object to be carbonitrided and simultaneously maintain a veryhard center. Carbonitriding at very high temperatures is impractical because of the very rapid thermal decomposition of the nitrogenous portions of the carbonitriding gas. Carbonitriding of nodular iron or gray iron substantially below 1500" F. produces inferior results by virtue of the premature decomposition of austenite into ferrite. These facts determine that a suitable carbonitriding range is just above 1500 F. and below the temperatures at which very rapid decomposition of the nitrogen compounds in the carbonitriding atmosphere takes place.

Accordingly, to accomplish such carbonitriding, the gray iron or nodular iron object is heated sufliciently high to completely austenitize the object. It is then cooled to a temperature just above 1500 F. and exposed at this temperature to a carbonitriding atmosphere. The object is then quenched and, if necessary, tempered to the desired value.

I claim as my invention:

A process for carbonitriding a metal chosen from the group consisting of nodular iron and gray iron comprising completely austenitizing the metal, cooling the austenitized metal from the austenitizing temperature to a selected temperature which selected temperature is high enough to insure substantially complete stability of the austenitic structure during the carbonitriding cycle, which selected temperature is low enough to insure substantial stability of the carbonitriding atmosphere and which selected temperature is not substantially lower than 1500" F., carbonitriding the metal at the selected temperature to produce a very hard case, and quenching the carbonitrided metal to produce a hard core.

References Cited in the file of this patent UNITED STATES PATENTS 946,360 Hodgkinson Ian. 11, 1910 1,065,697 Machlet June 24, 1913 1,092,925 Machlet Apr. 14, 1914 1,748,623 Sergeson Feb. 25, 1930 2,025,134 Stammberger Dec. 24, 1935 2,151,190 Cowan Mar. 21, 1939 2,188,226 Machlet Jan. 23, 1940 2,472,320 Vennerholm June 7, 1949 FOREIGN PATENTS 696,688 Great Britain Sept. 9, 1953 OTHER REFERENCES W. H. Holcroft: Carbonitriding in Present Practice," Metal Progress, vol. 58, No. 6, December 1950, pp. 843- 846.

G. W. Rengstorif et al.: The Carbonitriding Process of Case Hardening Steel, ASM preprint, 1950, No. 1, 27 pp. 

